The present invention relates to fluid couplings, and in particular to couplings which are used in high pressure hydraulic environments, as encountered on machine tools, agricultural and construction equipment and the like.
For many years, the hydraulics industry has relied on hydraulic couplings for use in manifolds, pump housings and control units, that comply with the SAE J514 standard. These couplings utilize an O-ring, and are known in the industry as O-ring boss fittings. Although the O-ring boss fitting is significantly better than the metal-to-metal couplings used elsewhere in hydraulic systems, and functions well in lower pressure environments, current hydraulic applications utilize significantly higher pressures, and the application of hydraulics to the robotics and other environmentally sensitive applications dictates that all hydraulic couplings be leak free.
A major reason for the failure of certain prior art O-ring fittings is the rotational loosening of threads which occurs under conditions of high pressure and vibration. During this movement, a gap between the back face of the hydraulic fitting and the front face of the hydraulic port is opened up, which eventually leads to the O-ring material being extruded under pressure into the interface. FIG. 1 illustrates a prior art O-ring boss fitting wherein the manifold 12 has a port 14 therein which is provided with internal threads 16 that are threadedly connected to the external threads 18 of fitting 20. As will be seen, O-ring 22 is received within an undercut groove 24 in fitting 20 and fills substantially all of the space therein. As the fitting 20 is screwed into port 14, O-ring 22 is compressed and becomes, in all respects, another solid component, incapable of responding to pressure differentials within the interface area. Even in the case where the O-ring may not fill the entire space but fills a very high percentage of the space, the O-ring will be incapable of experiencing any significant axial movement and will function similarly to the case where the O-ring substantially fills the entire space.
FIG. 2 illustrates what happens when fitting 20 is subjected to conditions of pressure and vibration. As will be seen, the rotation of threads 16 and 18 creates a gap 26 between hexagonal collar 28 of fitting 20 and the face 30 of manifold 12. As hydraulic pressure is exerted on O-ring 22, it is urged outwardly and a portion 32 thereof will be extruded through gap 26. When the hydraulic pressure is subsequently released, the extruded portion 32 of the O-ring is nibbled by the abutting faces of the fitting 20 and manifold 12 as they settle back together. This occurs repeatedly upon application and relaxation of pressure, until failure eventually occurs. It is undesirable for the O-ring to be under high compressive forces, because this causes it to function as a packing material that is not capable of significant axial movement within the space.
Other prior art techniques have been attempted in an effort to provide improved hydraulic sealing. For example, the European industry has utilized bonded seals, which comprise a metal washer to which is bonded a rubber washer of special cross-sectional shape, wherein the metal washer provides a backup to the rubber sealing member. Although such bonded seals work effectively in many applications, they are relatively expensive to manufacture, and the seals are difficult to use, often resulting in poor assembly and subsequent field failure. Another attempted solution is the use of O-ring boss components which are made with additional threads, whereby the additional friction afforded helps to resist rotational loosening of components. This is an additional cost, and has not proved to be as effective as the industry demands. A further type of prior art seal comprises an O-ring having a resilient spring as its core covered with a softer outer layer. This type of O-ring is not readily deformable and is designed to seal through the action of radial forces caused by an interference fit between the O-ring and the surfaces between which it is seated.
In prior art fittings, the O-ring seal is typically located between the hydraulic fluid within the system and the threads of the hydraulic fitting, thereby exposing the threads to the ambient atmosphere. This leaves the threads exposed to possible corrosion by electrolytic action and atmospheric attack, thereby making it difficult to disconnect the fitting. Should this corrosion occur at the interface between the hydraulic conveyance system and the manifold to which it attaches, it would be impossible to disassemble the system for repair or maintenance without damaging expensive components, such as the pumps and controls into which the connecting ports are machined. The problem of thread corrosion can be eliminated by locating the threads in communication with the hydraulic fluid. However, a major reason for the failure in service of an O-ring boss fitting is the loosening of the threads which occurs under conditions of pressure and vibration, and this situation is worsened by the lubrication of the threads by the hydraulic fluid within the system.
Current standard fittings include a thread which mates with the port entry thread, the former being undercut to accept the O-ring. Behind the O-ring there is a flat face metal-to-metal abutment, with the interface being perpendicular to the axis of the fitting. This face-to-face abutment does not lock the components together, and under conditions of vibration in combination with pressure, there is a loosening which takes place between the fitting and the port, which leads to failure of the coupling. A similar situation is present in the adjustable version of the prior art fitting, wherein a lock nut and washer provide the backup to the O-ring. The adjustable version of the standard fitting is even more prone to failure than the straight version, due to the fact that under pressure, the washer tends to deform slightly, thereby allowing a small gap to occur between its inner diameter and the outer diameter of the thread undercut against which it should abut with an interference fit. This creates two possible leak paths and a greater incidence of failure.